Photolithography is an integral process in manufacturing semiconductor IC (integrated circuit) devices. In general, a photolithographic process includes coating a semiconductor wafer (or substrate) with a layer of photoresist, and exposing the photoresist with an actinic light source (such as an excimer laser, mercury lamp, etc.,) through a photomask having an image of an integrated circuit. For example, a lithographic tool, such as a deepUV stepper can be used to project light through a photomask and a high aperture lens onto a photoresist layer, where the light intensity casts the photomask pattern onto the photoresist Various types of photomasks have been developed for lithography including, binary masks, embedded attenuated phase shift masks (EAPSM), alternating aperture phase-shift masks (AAPSM), as well as various hybrid mask types.
Currently, highly integrated circuit (IC) devices are being designed with IC device features having small critical dimensions. The critical dimension (CD) refers to the smallest width of a line or the smallest space between two lines that is specified according to design rules for a given device fabrication process. In fact, IC devices are currently being built with sub-wavelength feature sizes, where the circuit images printed on the silicon wafer are smaller than the wavelength of the light source used to expose the pattern. For example, state of the art DUV steppers use an argon fluoride (ArF) laser with a wavelength of 193 nm to form integrated circuits with feature sizes of 100 nm (0.1 micron) and below.
As feature patterns become increasing smaller (e.g., subwavelength features), however, it becomes increasingly difficult to meet critical dimension requirements as a result of optical proximity effects (OPE) which decrease the lithographic process window for printing sub-wavelength features. The OPE phenomenon occurs due to diffraction of light waves of closely spaced, adjacent circuit features which causes the light waves to interact in a way to distort the transferred pattern features and produce pattern-dependent process variations. In this regard, various techniques have been developed to mitigate or compensate for the effects of OPE when printing sub-wavelength features.
For example, well-known reticle enhancement techniques, such as optical proximity correction (OPC) and phase shift mask (PSM) techniques, are used for constructing photomasks. With OPC, small sub-resolution (nonprinting) features (such as “scatter bars”) are incorporated in circuit mask patterns to compensate for proximity effects. Further, PSM techniques are used to construct photomasks (e.g., alternating aperture phase-shift masks, embedded attenuated phase-shift masks, etc.) having mask patterns with phase-shifting structures designed to reduce proximity effects and enhance the contrast at critical edges of sub-wavelength features. On the other hand, as compared to PSM structures, binary masks are generally known to be more susceptible to OPE due to diffraction, which limits the ability to use binary masks for lithographic printing of sub-wavelength features.
FIGS. 1A, 1B and 1C schematically illustrate a conventional photolithography process using a binary mask structure. In particular, FIG. 1A is a top plan view of a binary photomask (10) and FIG. 1B is a schematic cross-sectional view of the binary photomask (10) along line 1B-1B in FIG. 1A. In general, the binary mask (10) comprises a mask pattern (11) formed on a mask substrate (12). The mask substrate (12) is formed of a material that is transparent to exposure light of a given wavelength of the exposure light. For example, the substrate (12) is typically formed of high-purity quartz or glass. For a binary mask, the image pattern (11) is typically formed of a light blocking material such as chromium (Cr) having a transmittance of about 0% at the given wavelength which operates to block (and reflect) the passage of light. In this regard, a binary mask is deemed a reflective mask.
In FIGS. 1A and 1B, the mask pattern (11) includes a pluratity of elongated parallel line features (11a) with pitch P, and spaces (11b) formed by etching a layer of light blocking material (e.g., Cr) on the mask substrate (12). The mask pattern (11) can be transferred to a photoresist layer on the substrate through a lithographic process. In particular, as depicted in FIG. 1B, during an exposure process, light of a given wavelength incident on the patterned surface of the mask (10) can be projected through the exposed regions (e.g., spaces 11b) of the photomask (10) onto a photoresist (resist) coated wafer such that the regions of the photoresist aligned to the spaces (11b) are exposed to light. With a positive resist, for example, the exposed regions of the photoresist can be removed during development such that mask pattern (11) is printed in the photoresist.
As the critical dimensions of the features to be printed become smaller and approach the resolution of the lithography exposure tool, the ability to accurately print small features using binary mask techniques, per se, is significantly reduced due to optical proximity effects due to diffraction. This limitation is schematically illustrated in FIG. 1C. In particular, FIG. 1C illustrates a semiconductor device (14) including a photoresist layer (15) formed on a semiconductor substrate (16) (e.g., wafer). In FIG. 1C, it is assumed that the photoresist layer (15) is a “positive resist” exposed using the binary mask (10) of FIGS. 1A and 1B with 1× reduction. It is further assumed that the critical dimensions of the line features (11a) and spaces (11b) to be printed are close to the resolution limit of the exposure system.
As depicted in FIG. 1C, the optical proximity effects due to the closely spaced line features (11a) prevents the line-space patterns from being printed in the photoresist (15). In particular, FIG. 1C depicts the electric field curve (13) (magnitude and direction) in the wafer plane across the photoresist (15) due to diffraction effects. In particular, because of the small sizes of the line and space features (11a, 11b), diffraction effects of incident light on the photoresist (15) causes the electric field vectors of adjacent space features (11b) to interact and add constructively such that the light intensity increases at regions of the photoresist (15) aligned to the line features (11a). FIG. 1C illustrates a circumstance in which the electric field (13) meets or exceeds the photoresist exposure threshold Tp across the entire region of the photoresist aligned to the line-space pattern (11a, 11b). As a result the line features (11a) are not printed and the space features (11b) are printed in the photoresist (15) as a single, wide space feature rather than discrete space features. These diffraction effects can be mitigated using PSM techniques.
For instance, FIGS. 2A, 2B and 2C schematically illustrate a conventional photolithography process using an EAPSM (Embedded Attenuated Phase Shift Mask) structure. In particular, FIG. 2A is a top plan view of an EAPSM structure (20) and FIG. 2B is a schematic cross-sectional view of the EAPSM structure (20) along line 2B-2B in FIG. 2A. In general, the EAPSM structure (20) comprises a mask pattern (21) formed on a mask substrate (22). The mask substrate (22) is formed of a material, such as high-purity quartz or glass, which is transparent at a given wavelength of the exposure light. The mask pattern (21) is formed of a light blocking material (or phase-shift material), such as molybdenum silicide (MoSi), having a transmittance in a range of 2-10%, at the given wavelength. FIGS. 2A and 2B depict a mask pattern (21) that includes a plurality of elongated parallel line features (21a) with pitch P, and spaces (21b), similar to the line-space mask pattern of FIGS. 1A/B. As compared to the photomask (10) of FIGS. 1A/B, the photomask (20) in FIG. 2A/2B causes DUV destructive interference at the wafer level, which enables line features to be more accurately printed as subwavelength dimensions smaller than the wavelength of light. This is conceptually illustrated in FIG. 2C.
In particular, FIG. 2C illustrates a semiconductor device (24) including a photoresist layer (25) formed on a semiconductor substrate (26) (e.g., wafer). In FIG. 2C, it is assumed that the photoresist layer (25) is a “positive resist” exposed using the binary mask (20) of FIGS. 2A/2B with 1× reduction. FIG. 2C depicts a resulting electric field curve (23) (magnitude and direction) in a wafer plane across the photoresist (25). The line features (21a) allow a small percentage of incident light to pass through the mask substrate (22) to the photoresist, but the intensity of such light is weak and does not expose the resist (25) on the wafer (26). The mask line features (21a) induce a 180 degree phase-shift of light passing through the mask (20) as compared to light that passes through the mask (20) at exposed regions of the substrate (22) (at space features (21b), which increases the image contrast at critical edges of the mask features and, thus, increase the resolution of the lithography process. More specifically, as depicted in FIG. 2C, destructive interference occurs at the critical edges of the line features (21a) adjacent the glass. In this regard, the electric field intensity is maintained well below the resist threshold Tp at regions of the photoresist (25) aligned to the mask line features (21a), enabling increased resolution for printing line-space patterns with subwavelength CDs using currently available lithographic tools.
Alternating aperture is another PSM technique that relies on DUV destructive interference to reduce the effects of OPE and print sub-wavelength features. For example, FIGS. 3A, 3B and 3C schematically illustrate a conventional photolithography process using an AAPSM (Alternating Aperture Phase Shift Mask). In particular, FIG. 3A is a top plan view of an AAPSM structure (30) and FIG. 3B is a schematic cross-sectional view of the AAPSM structure (30) along line 3B-3B in FIG. 3A. In general, the AAPSM structure (30) comprises a mask pattern (31) formed on a mask substrate (32). The mask substrate (32) is formed of a material, such as high-purity quartz or glass, which is transparent at a given wavelength of the exposure light. The mask pattern (31) is formed of a light blocking material such as chromium (Cr) having a transmittance of about 0% at the given wavelength which operates to block (and reflect) the passage of light. FIGS. 3A and 3B depict a mask pattern (31) that includes a plurality of elongated parallel line features (31a) with pitch P, and spaces (31b), similar to the line-space mask pattern of FIGS. 1A/1B. As compared to the photomask (10) of FIGS. 1A/1B, the photomask (30) in FIG. 3A/3B further includes trenches (32a) that are selectively etched into the mask (quartz) substrate (32) in every other one of the space features (31b). The trenches (32a) generate a 180 degree phase shift relative to those regions of the mask substrate that are not etched. The resulting phase differences lead to DUV destructive interference, which improves image contrast. This is conceptually illustrated in FIG. 3C.
In particular, FIG. 3C illustrates a semiconductor device (34) including a photoresist layer (35) formed on a semiconductor substrate (36) (e.g., wafer). In FIG. 3C, it is assumed that the photoresist layer (35) is a “positive resist” exposed using the binary mask (30) of FIGS. 3A/3B with 1× reduction. FIG. 3C depicts a resulting electric field curve (33) (magnitude and direction) in a wafer plane across the photoresist (35). The space features (31b) allow incident light to pass through the trash substrate (32) to the photoresist, whereas the line features (31a) reflect light. The trenches (32a) induce a 180 degree phase-shift of light passing through the mask (30) as compared to light that passes through the mask (30) though the exposed, unetched regions of the substrate (32) at space features (31b). As a result, the electric field (33) will be of equal magnitude and opposite phase on opposite sides of the line features (31a) and destructive interference occurs in the transitions between the etched and unetched regions produces a dark area that enhances the image contrast for printing the line-space features (31a, 31b) in the resist (35) with high precision.
Although PSM techniques discussed above can be generally used to provide increased resolution for printing sub-wavelength features, the quality with which such features can be replicated lithographically depends primarily on the size of the lithographic process window. In general, as is well known in the art, the term “process window” refers the amount of variation in exposure dose and focus which can be tolerated so that the characteristics of printed photoresist features (e.g., line width, wall angle, resist thickness) are maintained within prescribed specifications. For a given lithographic environment, the sensitivity of such photoresist features to changes in exposure dose and focus can be determined experimentally (or through computer simulations) by obtaining a matrix of focus-exposure data. For instance, for a given lithographic process and mask, the data of a focus-exposure matrix data can be used to determine variation of line width as a function of focus and exposure dose.
FIG. 4A is an exemplary Bossung (focus-exposure) plot which includes parametric curves of line width (CD) versus focus with exposure dose as a parameters. In, particular, the exemplary Bossung Plot illustrates the variation in CD (y-axis) as a function of defocus (x-axis) at different exposure energies (E1˜E5). In FIG. 4A, a dotted line (40) denotes a target (nominal) CD and dotted lines (41) and (42) respectively represent the acceptable upper (CD+) and lower (CD−) values, which vary from the target CD (40). The defocusing parameter (X-axis) denotes a relative deviation from a best focus position. In FIG. 4A, the best focus position is depicted as defocusing=0.
A lithographic process will be deemed robust if large variations in focus and dose minimally impacts the target CD (40) (maintaining the printed CDs within a desired range of acceptable CDs). In particular, a usable process window can be specified as the combination of DOF (depth of focus) and exposure latitude (LE) that maintains printed features within +/−10% of a target CD. The term exposure latitude (EL) denotes a percentage dose range of exposure energies (usually expressed as a percent variation from the nominal) that keeps the CD within specified limits. The usable focus range or depth of focus (DOF) typically refers to the range of focus settings wherein the lateral dimension of the printed feature or the space between features lies within a specification which is typically +/−10% of a targeted line width or CD. The concept of DOF is schematically illustrated in FIG. 4B.
In particular, FIG. 4B illustrates a lithographic projection process using a reticle to expose a photoresist coated substrate. In particular, FIG. 4B is a high-level schematic illustration of a projection system comprising a light source (43), a condenser lens (44), reticle (45) and projection lens (46). The light source (43) emits light which is incident on the condensing lens (44). The light passes through condensing lens (44) and evenly irradiates the entire surface of reticle (45) on which a predetermined pattern is formed. Thereafter, light passing through the reticle (45) is reduced by a predetermined scale factor via the projection lens (46) and exposes a photoresist layer (47) on semiconductor substrate (48). By using the projection optics (46), the size of mask features on the reticle (45) are typically 4 or 5 times larger than the same feature which is printed in the photoresist (47). For example, a mask line feature with a 1 micron width on the reticle would translate to a 0.2 micron wide line printed in the photoresist in a 5× reduction projection system.
FIG. 4B conceptually illustrates DOF. In general, the focal plane of the optical system is the plane which contains the focal point FP. The focal plane is typically referred to as the plane of best focus of the optical system The term focus refers to the position of the plane of best focus of the optical system relative to a reference plane, such as the top surface of the resist layer or the center of the photoresist, as measured along the optical axis (i.e., perpendicular to the plane of best focus). For instance, as depicted in FIG. 4B, the plane of best focus (focal plane) is placed near the surface of the photoresist layer (47). In the exemplary embodiment of FIG. 4B, focus is set by the position of the surface of the resist layer (47) relative to the focal plane of the imaging system. The term defocus refers to the distance, measured along the optical axis (i.e., perpendicular to the plane of best focus) between the actual position of the reference plane of the resist-coated wafer (e.g., the surface of the resist layer (47)) and the position if the wafer were at best focus. During a photolithographic process, the focus can change from the best focus to +/− defocus position. The DOF refers to the acceptable range of +/− defocus.
Referring again to FIG. 4A, variations in focus and exposure dose can lead to an increase or decrease of the CDs of printed features (from the target CD) outside the acceptable range of CDs. In general, a narrow process window will be realized if the line width drastically changes as a function of focus change. For example, as depicted in FIG. 4A, the parametric curves E1, E2, E4 and E5 illustrate that for the corresponding exposure doses, CD is more sensitive to deviations in focus from the best focus position (defocus=0). In contrast, the curve E3 is more linear, which indicates that for the given exposure dose, CD is less sensitive to deviations in focus from the best focus position (defocus=0).
Although enhancement techniques such as AAPSM and EAPSM discussed above can be utilized to improve resolution, such techniques can be complex, costly and can require increased chip size. Moreover, PSM technology is subject to the “forbidden pitch” phenomenon, resulting in reduced process windows. More specifically, with off-axis illumination, for a given feature and target CD, there can be one or more pitches where the process latitude of a dense pattern of such feature may be worse than that of an isolated feature of the same size. When the off-axis illumination is optimized for a given pitch (e.g. the smallest pitch on the mask), there may be pattern having a pitch where the angle of the illumination together with the angle of diffraction results in diffraction that yields a reduced DOF for that pitch. The forbidden pitch phenomenon has become a limiting factor in advanced photolithography for printing sub-wavelength features.
Exposure tools have a “focus budget” which refers a minimum DOF requirement of a photolithography process that is required to cover focus variations of the exposure tool. If the DOF of a given layout pattern pitch is not greater than the focus budget required by the exposure tool, the layout pattern pitch is considered a forbidden pitch. As such, the ability to mitigate the forbidden pitch phenomenon will generally improve the CDs and process latitude obtainable utilizing current semiconductor device manufacturing tools and techniques.
When printing sub-wavelength features, it is important to control CD uniformity. However, minor variations in parameters of the exposure process on photolithographic exposure equipment (scanners/steppers), may cause the critical dimensions (CD) of printed features to fall outside an acceptable manufacturing tolerances. For example, the DOF is generally viewed as one of the most critical factors in determining the resolution of the lithographic projection apparatus. During a photolithographic process, the focal point of the exposure system can drift above or below the desired reference surface of the photoresist coated substrate due to, e.g., temperature or pressure drifts, wafer flatness variations or other factors. Depending on the process widow, the amount of focus shift (or defocus) from best focus can have a dramatic effect on the size of the printed feature. As such, it is highly desirable to be able to control the process such that the focus is kept within a usable range for each wafer. In this regard, the amount of defocus cannot be determined without an adequate method of measuring best focus.
In view of the above, it would be highly desirable to develop mask techniques and OPC solutions to improve lithographic process windows and increase the resolution of current optical exposure systems for precision printing sub-wavelength features. Moreover, given the sensitivity of CD variation with regard to focus drifts in sub-wavelength lithography processes, it would be highly desirable to develop techniques for efficiently detecting focal point drifts (magnitude and direction) during a photolithographic process and enable automated control of an exposure tool to adjust focal point and achieve CD uniformity.